[Mesa-dev] [PATCH 3/3] i965/gen9+: Switch thread scratch space to non-coherent stateless access.

[Kristian Høgsberg]

On Wed, Nov 25, 2015 at 11:37 AM, Francisco Jerez <currojerez at riseup.net> wrote:
> The thread scratch space is thread-local so using the full IA-coherent
> stateless surface index (255 since Gen8) is unnecessary and
> potentially expensive.  On Gen8 and early steppings of Gen9 this is
> not a functional change because the kernel already sets bit 4 of
> HDC_CHICKEN0 which overrides all HDC memory access to be non-coherent
> in order to workaround a hardware bug.
>
> This happens to fix a full system hang when running any spilling code
> on a pre-production SKL GT4e machine I have on my desk (forcing all
> HDC access to non-coherent from the kernel up to stepping F0 might be
> a good idea though regardless of this patch), and improves performance
> of the OglPSBump2 SynMark benchmark run with INTEL_DEBUG=spill_fs by
> 33% (11 runs, 5% significance) on a production SKL GT2 (on which HDC
> IA-coherency is apparently functional so it wouldn't make sense to
> disable globally).

Very nice, all looks good to me. All three patches:

Reviewed-by: Kristian Høgsberg  <krh at bitplanet.net>

> ---
>  src/mesa/drivers/dri/i965/brw_eu.h               |  2 ++
>  src/mesa/drivers/dri/i965/brw_eu_emit.c          | 17 +++++++++++++++--
>  src/mesa/drivers/dri/i965/brw_vec4_generator.cpp |  4 ++--
>  3 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_eu.h b/src/mesa/drivers/dri/i965/brw_eu.h
> index 829e393..62891c7 100644
> --- a/src/mesa/drivers/dri/i965/brw_eu.h
> +++ b/src/mesa/drivers/dri/i965/brw_eu.h
> @@ -317,6 +317,8 @@ void brw_oword_block_read(struct brw_codegen *p,
>                           uint32_t offset,
>                           uint32_t bind_table_index);
>
> +unsigned brw_scratch_surface_idx(const struct brw_codegen *p);
> +
>  void brw_oword_block_read_scratch(struct brw_codegen *p,
>                                   struct brw_reg dest,
>                                   struct brw_reg mrf,
> diff --git a/src/mesa/drivers/dri/i965/brw_eu_emit.c b/src/mesa/drivers/dri/i965/brw_eu_emit.c
> index da1ddfd..bb6f5dc 100644
> --- a/src/mesa/drivers/dri/i965/brw_eu_emit.c
> +++ b/src/mesa/drivers/dri/i965/brw_eu_emit.c
> @@ -1997,6 +1997,19 @@ void gen6_math(struct brw_codegen *p,
>     brw_set_src1(p, insn, src1);
>  }
>
> +/**
> + * Return the right surface index to access the thread scratch space using
> + * stateless dataport messages.
> + */
> +unsigned
> +brw_scratch_surface_idx(const struct brw_codegen *p)
> +{
> +   /* The scratch space is thread-local so IA coherency is unnecessary. */
> +   if (p->devinfo->gen >= 8)
> +      return GEN8_BTI_STATELESS_NON_COHERENT;
> +   else
> +      return BRW_BTI_STATELESS;
> +}
>
>  /**
>   * Write a block of OWORDs (half a GRF each) from the scratch buffer,
> @@ -2097,7 +2110,7 @@ void brw_oword_block_write_scratch(struct brw_codegen *p,
>
>        brw_set_dp_write_message(p,
>                                insn,
> -                              255, /* binding table index (255=stateless) */
> +                               brw_scratch_surface_idx(p),
>                                msg_control,
>                                msg_type,
>                                mlen,
> @@ -2183,7 +2196,7 @@ brw_oword_block_read_scratch(struct brw_codegen *p,
>
>        brw_set_dp_read_message(p,
>                               insn,
> -                             255, /* binding table index (255=stateless) */
> +                              brw_scratch_surface_idx(p),
>                               msg_control,
>                               BRW_DATAPORT_READ_MESSAGE_OWORD_BLOCK_READ, /* msg_type */
>                               BRW_DATAPORT_READ_TARGET_RENDER_CACHE,
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp b/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
> index 20107ac..65b4358 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
> @@ -800,7 +800,7 @@ generate_scratch_read(struct brw_codegen *p,
>     if (devinfo->gen < 6)
>        brw_inst_set_cond_modifier(devinfo, send, inst->base_mrf);
>     brw_set_dp_read_message(p, send,
> -                          255, /* binding table index: stateless access */
> +                           brw_scratch_surface_idx(p),
>                            BRW_DATAPORT_OWORD_DUAL_BLOCK_1OWORD,
>                            msg_type,
>                            BRW_DATAPORT_READ_TARGET_RENDER_CACHE,
> @@ -873,7 +873,7 @@ generate_scratch_write(struct brw_codegen *p,
>     if (devinfo->gen < 6)
>        brw_inst_set_cond_modifier(p->devinfo, send, inst->base_mrf);
>     brw_set_dp_write_message(p, send,
> -                           255, /* binding table index: stateless access */
> +                            brw_scratch_surface_idx(p),
>                             BRW_DATAPORT_OWORD_DUAL_BLOCK_1OWORD,
>                             msg_type,
>                             3, /* mlen */
> --
> 2.5.1
>
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